Method of preparing a catalytic structure

ABSTRACT

A method of preparing a catalytic structure the method including the steps of: providing a solution of a precursor compound in a solvent at ambient conditions; providing a suspension of a support material having a specific surface area of at least 1 m2/g in a solvent at ambient conditions; mixing the solution of the precursor compound and the suspension of the support material; providing a reactive solvent in a supercritical or subcritical state; admixing the mixture of the solution of the precursor compound and the suspension of the support material in the supercritical or subcritical reactive solvent to form a reaction solution; injecting the reaction solution into a reactor tube via an inlet; allowing a reaction of the precursor compound in the supercritical or subcritical reactive solvent in the reactor tube to form the catalyst nanoparticles on the support material to provide the catalytic structure.

The present invention relates to a method of preparing a catalyticstructure. The catalytic structure comprises catalytic nanoparticlesformed on a support material. The catalytic nanoparticles may bemetallic nanoparticles or nanoparticles comprising catalytic metalcompounds. The catalytic structure may be employed in the catalyticconversion of compounds, such as in fuel cells or in industrialconversion of compounds.

PRIOR ART

In the field of catalytic conversion of compounds there is an ongoingdesire to provide catalytic structures capable of more efficientconversion. A general trend has been to employ substrates of highspecific surface area, such as carbon nanotubes (CNT), for deposition ofnanosized catalytic particles with the aim of controlling the size anddistribution of the particles on the support.

A specific area of relevance is the provision of catalytic structures infuel cells. The development of portable electronic devices strivestowards smaller devices typically having the same or higher powerrequirements, and the limited power density of conventional batteriesbecomes critical. Examples of such devices are microelectronic devicese.g. various microsensors, microengines, biomedical microsystems,microelectromechanical systems etc. The ideal power source for thesetypes of devices would have larger power densities than presently usedbatteries, re-chargeable capabilities and easy handling (whenrecharging). In general batteries are becoming inadequate with respectto the power requirements for portable electronics, and fuel cells, inparticular direct alcohol fuel cell (DAFC), may present an alternativeto batteries.

The principle of a fuel cell such as DAFC or PEMFC can roughly bedivided into three main elements; the polymer electrolyte membrane, thecatalyst/electrode assembly and the general system/cell structuring. Theelectrode assembly may consist of a gas diffusion layer (GDL) consistingof carbon paper with a microporous layer (MPL) on which the catalyticstructure is situated, e.g. platinum on carbon support, which providesthe catalytic conversion of the fuel to an electrical current.

Catalytic structures comprising CNT's with immobilised metalnanoparticles are known from the prior art. For example, Schlange et al.(Bei/stein J. Org. Chem., 7:1412-1420, 2011) provide a process for thecontinuous preparation of CNT-supported platinum catalysts in a flowreactor. In the process multiwalled CNT's (MWCNT) are initiallypre-treated by washing in HCl and HNO₃. After ultrasonication a platinumprecursor (H₂PtCl₆.6H₂O) is reacted in an ethylene glycol solvent, whichserves to reduce the platinum precursor and deposit platinumnanoparticles on the MWCNT. This process provided platinum particles inthe size range of 0.8 nm to 2.8 nm on the MWCNT.

Dong et al. (Carbon, 48: 781-787, 2010) produce graphene-supportedplatinum and platinum-ruthenium nanoparticles for use in fuel cells. Theprocess of Dong et al. involved the dispersion of graphene oxide powderin an ethylene glycol (EG) solution followed by addition ofhexachloroplatinic acid EG solution or hexachloroplatinic acid EGsolution also containing ruthenium chloride and allowing a reaction totake place under alkaline conditions. In alternative processes graphiteand carbon black were employed as carbon supports. The processesafforded formation of nanoparticles, e.g. smaller than 10 nm, on thesupport materials. However, the process was slow and may not be easilyscaleable, and furthermore, the size distribution of the nanoparticlesprepared was not detailed.

Supercritical synthesis of the catalyst particles provides an approachto allow control of the size of the deposited particles. For example, WO2005/069955 describes methods for preparing catalytic structures ofnanostructures, e.g. CNT's, with catalytic metallic nanoparticles, e.g.with diameters between 2 and 12 nm. The catalytic structures of WO2005/069955 can be configured to catalyse oxygen reduction or methanoloxidation in a fuel cell. The methods of WO 2005/069955 generallyinvolve mixing a precursor in a carrier, e.g. carbon dioxide, andtransforming the precursor to form a metal. The transformation of theprecursor can occur in the carrier or on the surface of a nanostructuresubstrate. The metal may be formed in the carrier and can then betransported to the surface of the nanostructure substrate in the carrierwhile the carrier is in supercritical fluid form. Alternatively, thetransformation may occur on the surface of the nanostructure substratewhile the carrier is in supercritical fluid form. The precursor is acomplex that contains the metal precursor and a ligand or moiety thatsolubilises the compound in the carrier.

U.S. Pat. No. 5,789,027 discloses processes for chemical deposition ofthin films of material onto a substrate. In the process a precursor ofthe material is dissolved in a solvent under supercritical ornear-supercritical conditions and subsequently exposed to the substrateto the solution; a reaction reagent is then mixed into the solution andthe reaction reagent initiates a chemical reaction involving theprecursor, thereby depositing the material onto the substrate surface.The process of U.S. Pat. No. 5,789,027 does not allow formation ofnanoparticles on a support material, and the substrates employed in theexamples are all of macroscale.

US 2004/137214 discloses a method of manufacturing a material withsurface nanometer functional structure. The process comprises the stepsof providing a substrate and placing it in a high-pressure container;supplying a supercritical fluid into the high-pressure container; tuningthe temperature and pressure inside the high-pressure container to theirappropriate values; supplying a precursor of a target material to beformed with a surface nanometer functional structure to thehigh-pressure container; and releasing the pressure inside thehigh-pressure container after the fluid therein reaches its reactionbalance point, bringing the precursor to adhere on the substrate surfaceto form the surface nanometer functional structure.

WO 2006/080702 describes carbon nanotubes for a fuel cell and ananocomposite including the carbon nanotubes. For example, a method ofproducing a nanocomposite for a fuel cell is disclosed, which methoduses a supercritical CO₂ fluid deposition method, wherein a mesoporouscarbon support is mixed with a precursor of a metallic catalyst and themixture is reduced in a supercritical CO₂ fluid using hydrogen gas.

WO 2006/110822 provides processes for the preparation of a carbonaerogel supported catalyst, which may comprise metal particles having anaverage metal particle size of 2.5 nm or less. The structure of WO2006/110822 may be prepared by contacting a support with a metalprecursor dissolved in a supercritical fluid and reducing the metalprecursor to a metallic state either by thermal reduction or hydrogenreduction at proper conditions.

The supercritical treatments of WO 2005/069955 and WO 2006/110822 areperformed batchwise, which makes the synthesis troublesome to scale upfor industrial purpose. Furthermore, controlled high heating rates areproblematic to obtain in batchwise synthesis, leading to aninhomogeneous heating, and hence the resulting particles may not beoptimal for catalytic conversion processes.

As an alternative to batchwise processing Adschiri et al. (J. Am. Ceram.Soc., 75: 2615-18, 1992) introduced the concept of production ofparticles in a continuous supercritical reactor. It is demonstrated howparticles of AlOOH can be prepared from the precursor Al(NO₃)₃ insupercritical water. The reactor design of Adschiri et al. allowedcontinuous withdrawal of particles formed in the reactor. Adschiri etal. found that the temperature, pressure and precursor concentration hadan effect on particle size and morphology. The particles of Adschiri etal. are, however, microsized.

Hald et al. (Journal of Solid State Chemistry 179: 2674-2680, 2006)disclose the production of TiO₂ nanoparticles in a continuoussupercritical reactor. The particles are formed from a precursor oftitaniumisopropoxide, which is reacted in a mixture of supercriticalisopropanol and water (5%). Hald et al. show how homogeneousnanoparticles can be produced quickly due to the instantaneous formationof a large number of primary particles when the hot supercriticalsolvent meets the cold reactant. The nanoparticles of Hald et al. weregenerally in the range of 11 to 18 nm, and the particle size could becontrolled by varying temperature and pressure.

Kimura et al. (Colloids and Surfaces A: Physicochem. Eng. Aspects 231:131-141, 2003) disclose the preparation of platinum nanoparticles from aplatinum precursor (H₂PtCl₆.6H₂O and Na₂PtCl₆.H₂O) in sub- andsupercritical solvents. The solvents employed were water, ethanol, andtheir mixtures. The process of Kimura et al. required the presence ofpoly(N-vinyl-2-pyrrolidone) (PVP) as a protective polymer. All solventsallowed formation of platinum nanoparticles, which in some casesagglomerated to larger structures. Ethanol generally served as areducing agent, although when pure water was used as a solvent areducing effect was provided by decomposition of PVP. When sub- andsupercritical ethanol were employed as a solvent Kimura et al. foundthat the nanoparticles, which were of diameters of about 3 nm, tended toaggregate even to particles unable to pass a 50 μm filter. According toKimura et al. optimal production of platinum nanoparticles was providedin a subcritical 1:1 mixture of ethanol and water with a large molarexcess of PVP.

In light of the above there is a need for an improved method forproviding catalytic structures with catalytic nanoparticles. It is anaim of the present invention to address this need.

DISCLOSURE OF THE INVENTION

The present invention relates to a method of preparing a catalyticstructure, the method comprising the steps of:

providing a solution of a precursor compound in a solvent at ambientconditions;

providing a suspension of a support material having a specific surfacearea of at least 1 m²/g in a solvent at ambient conditions;

optionally sonicating the suspension of the support material;

mixing the solution of the precursor compound and the suspension of thesupport material;

providing a reactive solvent in a supercritical or subcritical state;

admixing the mixture of the solution of the precursor compound and thesuspension of the support material in the supercritical or subcriticalreactive solvent to form a reaction solution;

injecting the reaction solution into a reactor tube via an inlet

allowing a reaction of the precursor compound in the supercritical orsubcritical reactive solvent in the reactor tube to form the catalystnanoparticles on the support material to provide the catalyticstructure; and

withdrawing the catalytic structure from the reactor tube via an outlet.

The method of the invention thus provides a catalytic structure, wherethe catalytic effect of the structure is provided by catalystnanoparticles, e.g. of a metal in its metallic form or nanoparticlescomprising a metal compound, deposited on a support material. Thecatalytic structure may be suitable for any catalytic process that canbe catalysed via catalyst nanoparticles. In one embodiment the catalystnanoparticles are of a metal in its metallic form. In another embodimentthe catalyst nanoparticles comprise a metal compound, e.g. with a metalin an oxidised state. The catalyst nanoparticles are synthesisedcontinuously in a supercritical or subcritical solvent, which givesexcellent control of morphology, crystallinity, size and uniformity ofthe particles which are all important characteristics for catalyticproperties of the nanoparticles. In particular, the present inventorshave surprisingly found that the presence of a support material in thesynthesis allows that the catalyst nanoparticles can be formed directlyon the support material in the continuous process without agglomerationor precipitation of the formed particles. This further provides that thecatalyst nanoparticles can be distributed evenly on the supportmaterial, and that the spacing of the nanoparticles can be controlled.In a certain embodiment the size of the catalyst nanoparticles is in therange of about 1 nm to about 50 nm, and the nanoparticles are preferablymonodisperse. The catalytic structure can furthermore be prepareddirectly in a one-step reaction in the flow synthesis reactor so thatthe final catalytic structure can be withdrawn from the reactorrequiring only a minimum of additional processing steps, e.g. to purifythe catalytic structure. The method enables the use of environmentallyfriendly solvents in the continuous flow production of catalystnanoparticles, and offers laboratory-like control while providing highthroughput for larger productions and scalability for industrialapplication. The advantages of avoiding agglomeration also allow a moreefficient process with an increased yield from the expensive startingmaterials.

The catalyst nanoparticles are particles in the nanosize range, e.g.from about 0.1 nm to about 1000 nm, although it is also contemplatedthat the particles may be larger than nanosize, e.g. the particles maybe of microsize with a size within the range of about 1 μm to about 10μm.

In one embodiment, the catalyst nanoparticles are metallic and maycomprise any metal or mixture of metals known or expected to have acatalytic effect on a chemical reaction. Preferred metals for metalliccatalyst nanoparticles are platinum, ruthenium, gadolinium and yttriumand mixtures of platinum and ruthenium, gadolinium and/or yttrium. Inanother embodiment the catalyst nanoparticles comprise a metal compoundknown or expected to have a catalytic effect on a chemical reaction. Thecatalyst nanoparticles may comprise any catalytic metal compound.Catalytic metal compounds typically comprise a metal atom, e.g. atransition metal or a lanthanide, in an oxidised state and a partneratom, e.g. an atom from groups 13 (“the boron group”), 14 (“the carbongroup”), 15 (“the nitrogen group”) or 16 (“the oxygen group”) of theperiodic table of the elements or another ligand molecule, e.g. anorganic ligand or an inorganic ligand. The partner atom or ligand willtypically be in a reduced state. The catalytic metal compound maycomprise more than one metal atom, e.g. in a trace amount, and thecatalytic metal compound may comprise more than one partner atom orligand.

The metal catalyst nanoparticles are formed from a precursor compound.When the catalyst nanoparticles are metallic, the precursor compound maybe any metal salt or compound capable of forming a metal, i.e. a metalin its metallic form, following reaction, e.g. a reduction, in thereactive solvent. Preferred precursor compounds for providing metalliccatalyst nanoparticles are H₂PtCl₆.6H₂O platinum(II) acetylacetonate(Pt(C₅H₇O₂)₂) (also known as Pt(acac)₂), Ru(acac)₃ and RuCl₃. When thecatalytic nanoparticles comprise a metallic compound the precursorcompound may house the metal in a partly oxidised state and a partneratom in reduced state, e.g. fully reduced state, and further ligands oratoms corresponding to the difference in oxidation levels between themetal atom in its current and final states, e.g. (NH₄)₂MoS₄. Thus, anoxidation of the metal atom may provide the catalytic metal compound.The method of the invention may also employ more than one precursorcompound. For example, the method may employ two or more metalprecursors in order to provide metallic catalyst nanoparticlescomprising the corresponding two or more metals. The method may alsoemploy two or more precursor compounds for providing catalystnanoparticles with two or more metal compounds. Alternatively, a singlemetal compound may be prepared from two precursor compounds where onecontains the metal atom and the other contains the partner atom. It islikewise possible for the method to employ a mixture of one or moreprecursor compounds for providing metallic catalyst nanoparticles andone or more compounds for providing metal compound catalystnanoparticles in order to provide catalyst nanoparticles comprising amixture of a metal and a metal compound.

The support material may be any solid material of a high specificsurface area, e.g. in one embodiment the specific surface area is atleast 10 m²/g, in particular when the support is a carbon material.However depending on the intended use of the catalytic structure thespecific surface area may not be important. In some embodiments thespecific surface area is below 100 m²/g, e.g. from 1 to 10 m²/g or less.The support materials should be insoluble in the solvents employed inthe method of the invention, and the support material should alsogenerally be insoluble under the conditions of the intended catalyticprocess. Likewise, the support material may be chemically inert in themethod of the invention. In one embodiment a preferred support materialis a carbon material, e.g. graphene, carbon nanotubes (CNT), carbonblack or a carbon aerogel. Carbon materials are particularly preferredwhen an electron conducting support material is desired. In otherembodiments the support material is not electron conducting, and thesupport material may be selected inter alia on the basis of electronconductivity. The selection of a support material for a specificcatalytic structure is well-known to the skilled person.

In order to disperse the support material and optimise access to thelarge surface area of the support material, the method may comprise astep to improve the dispersion. Any technology allowing dispersion of aparticulate material may be used. For example, the suspension of thesupport material may be sonicated. The sonication may be performed atany stage prior to or during the step of admixing the mixture of thesolution of the precursor compound and the suspension of the supportmaterial in the supercritical or sub-critical reactive solvent. It isfurthermore possible to improve dispersion by including a dispersionagent in the suspension of the support material and/or the reactivesolvent. A preferred dispersion agent is ethylene glycol, e.g. at aconcentration of 1%. For carbon based support materials, improveddispersion can be provided by activating the carbon support material,such as by treating, e.g. stirring, in HNO₃ of high concentration, e.g.8 M, or H₂O₂, e.g. 2 M.

The method of the invention employs a solution of a precursor compoundin a solvent at ambient conditions, and a suspension of a supportmaterial in a solvent also at ambient conditions. Any solvent that isliquid at ambient conditions may be used in the method, and the solventfor dissolving the precursor compound and the solvent for suspending thesupport material may be the same or different. In a certain embodimentthe mixture of the solution of the precursor compound and the suspensionof the support material is prepared directly, e.g. by dissolving theprecursor compound and suspending the support material directly in thesame solvent. The reactive solvent may also comprise another component,e.g. other solvents or dissolved components, for example to activate orenhance the activation of the support material or improve dispersion ofthe support material. For example, a carbon support material may beactivated using 1 to 5% w/w of H₂O, H₂O₂, H₂SO₄, HNO₃ or a combinationthereof. The component to activate the support may also be provided witheither the solution of the precursor compound or the suspension of thesupport material. Activation of the support may improve the dispersionof the support material or the activation may improve formation of thecatalyst nanoparticles on the support, e.g. by improving physical orchemical binding of the catalyst nanoparticles or by providingnucleation points for formation of catalyst nanoparticles.

The solvents are selected so as to be soluble in the reactive solventunder supercritical or subcritical conditions. The solvents arepreferably the same and more preferably the same as the reactivesolvent. The solubilities of solvents in super- and subcriticalconditions are well-known to the skilled person. The concentrations ofthe precursor compound and the support material in the respectivesolvents may be chosen freely, although it is preferred that theconcentrations of the precursor compound and the support material are inthe range of about 1 wt % to about 10 wt %. The concentration may alsobe expressed in molar concentrations, and the concentration may be inthe range of 0.001 to 10 M, e.g. at 1 M or 0.1 M, althoughconcentrations outside these ranges are also contemplated.

Any reactive solvent allowing the precursor compound to form theappropriate catalyst compound, i.e. in the form of nanoparticles, whenthe reactive solvent is in a supercritical or subcritical state may beemployed in the method of the invention, and the reaction may be anychemical reaction allowing formation of the catalyst nanoparticles. Thereaction of the precursor compound in the reactive solvent may be areduction of a metal ion to convert the metal ion to the metal in itsmetallic form. Preferred reducing reactive solvents are ethanol,methanol, isopropanol, ethylene glycol and combinations thereof. It isfurther contemplated that water may serve as a reducing solvent. Whenthe precursor compound comprises a metal atom in a partly oxidised statethe reactive solvent may be an oxidising reactive solvent.

The reactive solvent is in a supercritical or subcritical state when itis admixed with the mixture of the solution of the precursor compoundand the suspension of the support material. For example, in oneembodiment the reactive solvent has a temperature at or within 100° C.below, or above the temperature of the critical point (T_(cr)) of thereactive solvent and the reactive solvent is at a pressure at or within30% below, or above the pressure of the critical point (P_(cr)) of thereactive solvent. When both the temperature and the pressure of thereactive solvent are above the respective values of the critical pointthe solvent is in a supercritical state. When either the temperature orthe pressure of the reactive solvent are below the respective values ofthe critical point but within the indicated ranges the solvent isconsidered to be in a subcritical state. Both of the temperature and thepressure of the reactive solvent may also be below the respective valuesof the critical point but within the indicated values; this is alsoconsidered to be a subcritical state in the present invention.

The mixture of the solution of the precursor compound and the suspensionof the support material may contain the precursor compound and thesupport material in any desired ratio. Likewise, the ratio of theprecursor compound, the support material and the reactive solvent mayalso have any desired value. A preferred ratio of precursorcompound:support material is 4:1. The mixture of the solution of theprecursor compound and the suspension of the support material may beprovided as a cold reaction line, e.g. at ambient conditions, which ismixed abruptly with the supercritical or sub-critical reactive solvent.Alternatively, the pressure and/or temperature of the mixture of thesolution of the precursor compound and the suspension of the supportmaterial may also be increased prior to admixing with subcritical orsupercritical solvent. For example, the mixture may be admixed withethanol as a reactive solvent, which is preheated at a pressure of ˜200bar resulting in a mixing temperature of ˜300° C. representing thesupercritical regime of ethanol. In a specific embodiment, the reactivesolvent is ethanol and the temperature is in the range of about 250° C.to about 400° C., and the pressure is in the range of about 100 bar toabout 300 bar. The step of mixing the solution of the precursor compoundand the suspension of the support material may be done before the stepof admixing the mixture of the solution of the precursor compound andthe suspension of the support material in the supercritical orsubcritical reactive solvent, or the solution of the precursor compoundand the suspension of the support material may be admixed simultaneouslywith the reactive solvent in a supercritical or subcritical state.Furthermore, either of the solution of the precursor compound or thesuspension of the support material may be brought to sub- orsupercritical conditions before admixing with the reactive solvent undersub- or supercritical conditions. High heating rates can be obtained bymixing the cold reaction line and the supercritical or subcriticalsolvent. The high heating rates can provide fast nucleation and reactionuniformity. In particular, the rapid increase in the temperature leadsto fast homogenous nucleation resulting in monodisperse nanoparticles,which are further matured in the heater before being cooled down. Thecritical temperature and pressure are solvent dependent, and hencetuneable by using different reactive solvents, e.g. in a pure form or asa mixture of solvents. The obtained product, i.e. the catalystnanoparticles, is tuneable by varying temperature and pressure, thuscontrollability of morphology, crystallinity, size, and uniformity ofthe particles are obtained. This results in homogenous nanoparticleswith a narrow, e.g. monodisperse, size distribution, which is crucialfor catalytic property of nanoparticles. The temperature and pressure ofthe reactive solvent may be controlled and varied throughout theprocess. For example, the reactive solvent may be at one set oftemperature and pressure upon admixing with the mixture of the solutionof the precursor compound and the suspension of the support material,and subsequently the temperature and pressure may be increased ordecreased in the reactor tube. The support material that is present inthe super- or sub-critical media prevents the catalyst nanoparticlesfrom agglomerating, as these attach directly onto the support material.

The method of the invention is performed in a reactor tube so that thereaction can be described as a continuous process, e.g. the reactiontakes place under continuous conditions. Operation under continuousconditions in a reactor tube provides advantages that cannot be realisedin a batch type operation. For example, the continuous operation allowsthat relatively small portions of the mixture of the solution of theprecursor compound and the suspension of the support material at a timeare admixed with the super- or subcritical reactive solvent ensuring afast and efficient change from ambient conditions to super- orsubcritical conditions at which the catalyst nanoparticles will form.This allows good control of the size and uniformity of thenanoparticles, and furthermore it allows that the nanoparticledistribution on the support material is controlled. In one embodimentthe catalyst nanoparticles are formed on the support material at aspacing between the catalyst nanoparticles which is in the range onabout 0.1 nm to about 100 nm. The combined control of size, uniformityand distribution on the support material cannot be achieved in a batchprocess.

The reactor tube has an inlet and an outlet. The step of admixing themixture of the solution of the precursor compound and the suspension ofthe support material in the supercritical or subcritical reactivesolvent in a reactor tube may thus be performed at the inlet of thereactor tube, e.g. in an injector or a mixing chamber, and the catalyticstructure may be withdrawn from the reactor tube at an outlet, so thatthe admixture, or “reaction solution”, will travel through the reactortube from the inlet to the outlet. For example, the admixture may traveldown a vertical reactor tube. It is preferred that the reactor tube isvertical with the inlet at an upper section of the reactor tube and theoutlet at a lower section of the reactor tube, so that the outlet isbelow the inlet. In another embodiment the inlet may also be below theoutlet so that the admixture travels upward in the reactor tube. Inother embodiments the reactor tube comprises one or more additionalinlets downstream of the first inlet. This allows for a more flexibleprocess, since for example it is possible to supply the reactionsolution with further precursor compounds allowing the formation ofcatalyst nanoparticles having a layered structure of different metals.

The reactor set-up may comprise a mixing chamber or the inlet tubes maycontain a static mixer to improve mixing. For example, the solution ofthe precursor compound and the suspension of the support material may bemixed using a static mixer prior to admixing with the reactive solvent.Likewise, the step of admixing the mixture of the solution of theprecursor compound and the suspension of the support material in thesupercritical or subcritical reactive solvent may be performed using astatic mixer. Static mixers are well-known to the skilled person. Thestep of admixing the mixture of the solution of the precursor compoundand the suspension of the support material in the supercritical orsubcritical reactive solvent may also be performed using cross-, vortex-or opposing flow-mixing.

The distance between the inlet and the outlet coupled with the flow rateof the admixture in the reactor tube provides a residence time for theadmixture flowing through the reactor tube. The residence time in thereactor tube allows that the particles are matured further to enhancecrystallinity, thereby generating more well-defined particles. The fluidmay be kept at supercritical or subcritical temperatures in the progressthrough the reactor tube, ensuring that all precursors may be used up.This provides better control of the process than is achievable in abatch process. A preferred residence time is in the range of about 2seconds to about 10 seconds. However, the residence time is generallydependent on the scale of operation, and the residence time may also beshorter than 2 seconds or higher than 10 seconds. For example, theresidence time may be 1 minute or more, such as 10 minutes or more. Itis noted that the flow of the reaction solution in the reactor tube mayalso be stopped, so that the flow can be described as astop-flow-operation; the flow does not need to be constant, and theflow-rate may be varied as desired.

The reaction solution of the mixture of the solution of the precursorcompound and the suspension of the support material in the supercriticalor subcritical reactive solvent may be cooled to liquefy the reactionsolution. The cooling may be performed at any stage in the process afterformation of the catalyst nanoparticles on the support material, and thereactor tube may comprise a cooling section between the inlet and theoutlet. The cooling may, e.g. be obtained indirectly by flowing water onthe outside of the reaction tube to a temperature where the reactionsolution liquefies. For example, the reaction solution may be exposed toa rapid cooling right before the exit via the outlet of the reactortube. The reactant or precursor inlets may also comprise a cooledsection, e.g. to prevent premature heating of the precursor compound.The pressure of the reactor tube may be relieved by a valve (Pressurerelease valve or back pressure regulator), and the reaction solution,including the catalyst nanoparticles synthesised directly onto thesupport material, can be continuously withdrawn or tapped.

The features of the embodiments described above may be combined freelyas desired, and embodiments from such combinations are also consideredwithin the scope of the invention.

In another aspect the invention relates to a catalytic structureobtainable in the method of the invention, preferably a catalyticstructure comprising platinum or platinum-ruthenium nanoparticles formedon a carbon support material, e.g. graphene, CNT's or carbon black. Thiscatalytic structure is suitable for a fuel cell, and in yet a furtheraspect the invention relates to a fuel cell comprising a catalyticstructure obtainable in the method of the invention. The fuel cell maybe a direct alcohol fuel cell, preferably a methanol or ethanol fuelcell, and the catalytic structure preferably comprises platinum orplatinum-ruthenium nanoparticles formed on a carbon support material,e.g. graphene, CNT's or carbon black. However, the fuel cell may also bea HTPEM, LTPEM, DFAFC, MCFC, Reformed methanol fuel cell, Phosphoricacid fuel cell.

BRIEF DESCRIPTION OF THE FIGURES

In the following the invention will be explained in greater detail withthe aid of an example and with reference to the figures, in which

FIG. 1a shows a schematic drawing of a continuous supercritical reactorset-up.

FIG. 1b shows a schematic drawing of a continuous supercritical reactorset-up with two separate reactant inlets.

FIG. 2a shows a schematic drawing of a continuous supercritical reactorset-up with two upstream reactant inlets and a downstream reactantinlet.

FIG. 2b shows a schematic drawing of a continuous supercritical reactorset-up with two upstream reactant inlets and a downstream reactant inletand a downstream solvent inlet.

FIG. 3a-e show electron micrograph images of an embodiment of theinvention.

FIG. 4 shows a powder X-ray diffraction (PXRD) of the catalyticstructure of the invention.

FIG. 5 shows a cyclic voltammetry (CV) plot of for catalytic structuresof the invention compared to a commercial catalytic structure.

FIG. 6 shows various mixing geometries for proper mixing of thereactants with the hot solvent string; (a) and (b) cross-mixing, (c)opposing flow-mixing, and (d) vortex-mixing.

FIGS. 7a and 7b show the correlation of the vertical heater temperature(T_(v)), pressure (P) and the Pt average particle size (PXRD measured).

FIG. 8 shows the correlation of size, ECSA and MA, of the Pt particlessynthesised on KetjenBlack (KB).

FIG. 9 shows Scanning Transmission Electron Micrographs (30 kV) showingcatalyst product synthesised at (a) T_(v)=250° C., p=300 bar, size=1.5nm and (b) T_(v)=400° C., p=300 bar, size=3 nm.

FIG. 10 shows the correlation of Pt:C ratio, ECSA and MA of Pt particlessynthesised on KB.

FIG. 11 shows Scanning Transmission Electron Micrographs showing Ptparticles synthesised onto various carbon supports; (a) and (b) Ptparticles on graphene with ratio 50:50 (Pt:G), (c) Pt particles onMWCNTs (8-13 nm diameter) with ratio 50:50 (Pt:CNT), (d) Pt particles onMWCNTs (8-13 nm diameter) with ratio 20:80 (Pt:CNT).

FIG. 12 shows thermogravimetric analysis (TGA) of three differentcatalysts with different carbon support nanomaterials.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of providing a solution of aprecursor compound in a solvent at ambient conditions;

providing a suspension of a support material having a specific surfacearea of at least 1 m²/g in a solvent at ambient conditions;

optionally sonicating the suspension of the support material;

mixing the solution of the precursor compound and the suspension of thesupport material;

providing a reactive solvent in a supercritical or subcritical state;

admixing the mixture of the solution of the precursor compound and thesuspension of the support material in the supercritical or subcriticalreactive solvent to form a reaction solution;

injecting the reaction solution into a reactor tube via an inlet

allowing a reaction of the precursor compound in the supercritical orsubcritical reactive solvent in the reactor tube to form the catalystnanoparticles on the support material to provide the catalyticstructure; and

withdrawing the catalytic structure from the reactor tube via an outlet.

In the context of the invention a “catalytic structure” comprises asupport material with catalyst nanoparticles, which particles maycatalyse a desired reaction. In one embodiment the catalystnanoparticles are metallic. Any catalytic metal may be relevant for thecatalytic structure, in particular transition metals. The metal may alsobe a mixture of two or more metals. The metal or mixture of metals maybe selected based on the reaction to be catalysed using the catalyticstructure. For example, the catalyst nanoparticles may be platinumparticles or platinum-ruthenium nanoparticles when the catalyticstructure is used in a direct alcohol fuel cell (DAFC). In general,metals of relevance comprise a transition metal, a lanthanide, Sc, Ti,V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Gd, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd,Pt, Au, Ir, W, Sr. The metal may also be a mixture of two or moremetals, such as Pt_(x)Ru_(y), Pt_(x)Y_(y), Pt_(x)Gd_(y), Pt_(x)Sc_(y),Pt_(x)Ti_(y), Pd_(x)Ti_(y), Pd_(x)Y_(y), Pt_(x)Nb_(y), Pt_(x)Zn_(y),Pt_(x)V_(y), Pt_(x)Cd_(y), Pd_(x)Cd_(y), Pt_(x)Cu_(y), Pd_(x)Cu_(y),Pd_(x)Nb_(y), Pd_(x)V_(y), Pt_(x)Mo_(y), Pt_(x)Fe_(y), Pt_(x)Cr_(y),Pd_(x)Cr_(y), Pt_(x)Ni_(y), Pt_(x)Co_(y), Pd_(x)Ni_(y), Pd_(x)Co_(y),Pt_(x)Mn_(y), Pt_(x)Rh_(y), Pt_(x)Ir_(y), Pt_(x)Ru_(y)Mo_(z),Pt_(x)Ru_(y)W_(z), Pt_(x)Ru_(y)Co_(z), Pt_(x)Ru_(y)Fe_(z),Pt_(x)Ru_(y)Ni_(z), Pt_(x)Ru_(y)Cu_(z), Pt_(x)Ru_(y)Sn_(z),Pt_(x)Ru_(y)Au_(z), Pt_(x)Ru_(y)Ag_(z), Pd_(x)Ru_(y). When two or moremetals are employed the ratio between the metals, i.e. as represented byx and y and z in the listed combinations of metals, may be selectedfreely. For example, one embodiment of the invention providesplatinum-ruthenium nanoparticles where platinum and ruthenium are in theratio of about 1:1.

In another embodiment the catalyst nanoparticles comprise a catalyticmetal compound, e.g. a metal compound comprising a metal atom in anoxidised state and a partner atom or ligand molecule. The metal ispreferably a transition metal. Partner atoms may be boron (B), carbon(C), silicon (Si), germanium (Ge), nitrogen (N), phosphorus (P), arsenic(As), antimony (Sb), oxygen (O), sulfur (S), selenium (Se), or tellurium(Te). Halogen partner atoms are also contemplated in the invention.Exemplary catalytic oxides comprise MgO, Co_(x)O_(y), Fe_(x)O_(y)Fe₂O₃/NiO, Y_(x)Fe_(y)O_(z), FeTiO₃, CuFe₂O₄, ZnFe₂O₄, ZrFe₂, CuZnFe₄O₄,Zr₄Sc₁Fe₁₀, TiO₂, CeO₂, ZrO₂, and the catalytic nanoparticles maycomprise any of these materials. Further catalytic metal compounds areMo_(x)S_(y), CoS_(x)—MoS₂, Fe_(1-x)S_(x) (x=0-0.125),Ni(Co)Mo_(1-x)W_(x)S₂ and all these metal compounds are relevant for theinvention. The invention is not limited to these metal compounds andothers are known to the skilled person.

The catalyst nanoparticles may also comprise a mixture of a metal in itsmetallic form and a metal compound; the mixture may be random or thecatalyst nanoparticles may comprise layers, e.g. distinct layers, of ametal and a metal compound. Layered catalyst nanoparticles may beprepared by initially forming a core particle and subsequently adding,e.g. via an inlet downstream in the reactor tube of the first inlet, asecond, different precursor compound to the super- or subcriticalreactive solvent and allowing the second precursor to react in thepresence of the core catalyst nanoparticles. When the catalystnanoparticles comprise more than one metal or more than one metalcompound the catalyst nanoparticles may contain a first metal or metalcompound representing the majority, e.g. more than 90% w/w, more than95% w/w or more than 99% w/w, of the mass of the catalyst nanoparticleand one or more minor components, e.g. a metal or a metal compound,present in e.g. less than 10% w/w, less than 5% w/w or less than 1% w/w,of the mass of the catalyst nanoparticle. In this case the catalystnanoparticle can be said to be “doped” with the minor component. Dopedcatalysts and the relative amount of their components are well-known tothe skilled person.

The catalytic structure prepared in the method of the inventioncomprises catalyst nanoparticles formed on the support material. In thecontext of the invention a “nanoparticle” is a particle smaller than 1μm, e.g. in the range of about 0.1 nm to about 1000 nm, with the rangesof from about 1 nm to about 100 nm, or 3 nm to 50 nm, being preferred.Other preferred ranges are from about 1 nm to about 10 nm, e.g. about 1nm to about 5 nm. The nanoparticles formed in the method may bemonodisperse having a narrow size distribution; samples of particleswith standard deviations up to 50%, e.g. <40%, <30%, <20%, <10%, e.g.<5%, in diameter are considered monodisperse. For example, according toone embodiment of the invention the nanoparticles are of about 5 nm orabout 6 nm in size with the standard deviation of the particle size ofone batch of nanoparticles being within 50% of 5 nm or 6 nmrespectively. It is noted, however, that the smaller the nanoparticles,the larger the acceptable variation of the diameters for thenanoparticles to be considered monodisperse. It is further noted thatthe size of the catalyst nanoparticles is generally not dependent on thesupport material, e.g. the specific surface area of the supportmaterial. For example, the size of the catalyst nanoparticles can becontrolled via the temperature and control of the reaction solution.However, the support surface can also affect the catalyst particle size.

The “support material” is a solid material, which may be inert regardingthe reaction to be catalysed by the catalytic structure, and which has ahigh specific surface area allowing a high mass transfer rate in thecatalysed reaction. Thus, the support material may have a specificsurface area of at least 100 m²/g, although it is preferred that thespecific surface area is at least 250 m²/g, 500 m²/g, 1000 m²/g, 1500m²/g, 2000 m²/g, at least 2500 m²/g, or at least 3500 m²/g. In aspecific embodiment the specific surface area is in the range of 10 m²/gto 3500 m²/g, e.g. about 50 m²/g to about 1500 m²/g, about 100 m²/g toabout 1000 m²/g. In general the higher the specific surface area thehigher the mass transfer rate provided by the catalytic structure.Materials with specific surface areas relevant to the invention may beporous, or the high specific surface area may be due to the supportmaterial being present in an appropriately sized particulate form, orthe specific surface area may be due to a combination of particle sizeand porosity of the support material. Determination of the specificsurface area is well known to the skilled person. In specificembodiments it is possible to modify the surface of the support, e.g. tomodify the hydrophilicity or hydrophobicity. For example a supportmaterial may be exposed to reducing or oxidising conditions prior to thereaction in which the catalyst nanoparticles are formed. For example,graphene oxide may be subjected to reducing conditions to providereduced graphene oxide. In particular, the reactive solvent may comprisea component to activate the support material.

Preferred support materials, especially when the catalytic structureshould have electron conductive properties, are carbon materials, suchas graphene, graphene oxide, reduced graphene oxide, carbon nanotubes(CNT), e.g. single-walled or multi-walled CNT's, bucky balls, carbonparticles, carbon black, e.g. Vulcan XC-72 and 72R from CABOT andKetjenBlack from Akzo Nobel; aerogels; ceramic materials; metals; metalalloys, zeolites, tungsten carbide, metal oxides such as Al₂O₃,γ-AlO(OH), TiO₂, MgO, La₂O₃, ZrO₂, SiO₂, SiO₂—Al₂O₃, SO₄ ²⁻—ZrO₂, CeO₂,ZnO, IrO₂, Cr₂O₃, MgAl₂O₄, BaSO₄, CaCO₃, SrCO₃ etc. The support materialmay be selected based on desired characteristics of the catalyticstructure prepared. For example, when the catalytic structure is anelectrode material for a fuel cell the support material preferably hashigh electrical conductivity. The specific surface areas of exemplarycarbon support materials are up to 250 m²/g for carbon black, about 250to about 1250 m²/g for CNT's (e.g. single-walled) and >2,000 m²/g forgraphene. Certain carbon black materials may also have specific surfaceareas outside these typical ranges, for example Akzo Nobel providescarbon black materials of about 1400 m²/g (KetjenBlack EC-600JD) andabout 800 m²/g (Ketjenblack EC-300J). Other relevant support materialsare any materials conventionally used in the field of heterogeneouscatalysis, as are known to the skilled person. Exemplary supportmaterials comprise ceramic materials, such as alumina, titania, silica,zirconia, metal oxides, metal sulphides or metals.

The support material is provided as a suspension, which is mixed withthe solution of the precursor compound. This should be understoodbroadly, and it is also contemplated that the support material and theprecursor compound, e.g. in a dry form, are added to a solvent tosuspend the support material and dissolve the precursor compound inorder to provide the mixture of the solution of the precursor compoundand the suspension of the support material. Thus, a specific embodimentof the method of the invention comprises the step of providing asuspension of a support material in a solvent at ambient conditions,which suspension contains a precursor compound. Likewise, the supportmaterial, e.g. in a dry form, may be added to a solution of theprecursor compound, or the precursor compound, e.g. in a dry form, maybe added to a suspension of the support material in order to provide themixture of the solution of the precursor compound and the suspension ofthe support material.

The catalytic structure provided in the method of the invention hascatalyst nanoparticles providing the catalytic function of the catalyticstructure. The support material may also provide effects to thecatalytic structure. For example, a reduced graphene oxide support maymake the catalytic effect resistant to CO-poisoning when the catalyticstructure comprises platinum nanoparticles. The catalyst nanoparticlesare prepared from a “precursor compound”. The precursor compound may beany metal salt or compound capable of forming a metal in its metallicform, or the precursor compound may allow formation of a catalytic metalcompound comprising a metal atom in an oxidised state and a partner atomor ligand In general, the same precursor compounds may be employed toform either metallic catalyst nanoparticles of catalyst nanoparticlescomprising a metal compound; the choice of reactive solvent allowscontrol of the final oxidation state of the metal component in thecatalyst nanoparticles, e.g. if the metal component will be at oxidationlevel 0, i.e. metallic, in the catalyst nanoparticles, or if the metalcomponent will be at a higher oxidation level to form a metal compound.It is preferred that the precursor compound is soluble in a solvent, andit is further preferred that the dissolved form of the precursorcompound provides a solubilised metal ion or a metal ion solubilised asa complex with one or more partner atoms or ligands. For example,hexachloroplatinate may be used as a precursor compound for formingmetallic platinum. The partner atom or ligand may be any molecule thatcan form a complex with the metal ion, and in particular the partneratom or ligand may be a molecule that can stabilise the metal ion, e.g.prevent spontaneous oxidation or reduction, and aid in solubilising themetal ion. The molecule may be a simple ion, e.g. chloride, or anorganic compound or ion. The precursor compound may also be anorganometallic compound containing a bond between a carbon atom and themetal atom. The partner atom or ligand may also be a molecule, whichwill form part of the catalyst nanoparticles. For example, MoS₄ ²⁻ maybe employed as a complex of molybdenum to form catalyst nanoparticles ofMoS₂. In one embodiment the precursor compound comprises a metal atom inan oxidised form, which may be reduced to form the metallicnanoparticles. In another embodiment the precursor compound comprises ametal atom in a partly oxidised form, which may be oxidised to formcatalyst nanoparticles together with one or more partner atoms orligands. Alternatively, the precursor compound comprises a metal atom ina more, e.g. fully, oxidised form, which may be reduced to form catalystnanoparticles together with one or more partner atoms or ligands.Exemplary precursor compounds comprise A₃[VS₄], A₃[NbS₄], A₃[TaS₄],A₂[MoSe₄], A₂[WS₄], A₂[WSe₄], A[ReS₄], where A may be an alkali metalcation, [PPh₄]⁺, [NEt₄]⁺ ammonium or the like.

Certain embodiments of the method of the invention employ more than oneprecursor compound, which may be provided in a single solution, orindividual precursor compounds may be provided as separate solutions,which may be mixed prior to or simultaneously with the mixing with thesuspension of the support material. When multiple precursor compoundsare employed the ratio between the metal ions, e.g. expressed in termsof mass or molarity, may be chosen freely. In one embodiment theprecursor compound is H₂PtCl₆.6H₂O, Pt(acac)₂, Ru(acac)₃ or RuCl₃ or acombination thereof is employed. These precursor compounds allowformation of metallic platinum and ruthenium, respectively. Thesecompounds are soluble in an appropriate solvent, e.g. ethanol or wateror a mixture of ethanol and water, to prepare solutions that can bemixed with the reactive solvent in a supercritical or sub-criticalreactive solvent.

The ratio of the precursor compound to the support material, e.g.expressed as the mass of the metal component of the precursor compoundto the mass of the support material, may in the range of about 1:100 toabout 100:1, e.g. about 1:10 to about 10:1, about 1:5 to about 5:1,about 1:3 to about 3:1, about 1:2 to about 2:1 or about 1:1. Preferredratios in embodiments where metallic platinum or ruthenium are preparedon carbon support materials are 5% w/w to 50% w/w of carbon support toplatinum or ruthenium; in particular about 20% w/w, e.g. of platinum tographene, will give superior results for a catalyst structure regardingperformance as an electrode for a fuel cell. The concentrations of theprecursor compound and the support material in their respective solventsis preferably in the range of about 1 wt % to about 10 wt %, or 0.001 to1 M, e.g. 0.1 M. The volumes of the solution of the precursor compoundand the suspension of the support material are selected to provide thedesired ratio of the precursor compound and the support materialdepending on their respective concentrations. It is, however, preferredthat the volumes are of comparable size in order to ensure efficientmixing, and the concentrations will generally be selected to allowmixing of volumes of comparable size.

The method of the invention comprises steps where solvents are under“ambient conditions”. In the context of the invention the term “ambient”should be understood broadly and in particular it means that thepressure is not increased or decreased relative to the pressure of thesurroundings. The solvent under ambient conditions will be liquid, andfor certain solvents the temperature may be decreased or increasedrelative to the temperature of the surroundings, in particular in orderto ensure that the solvent is in a liquid state.

In terms of the present invention a “reactive solvent” is a solvent thatmay form a supercritical or subcritical state, and which furthercomprises a reactive compound that may react with a precursor compoundto form a metal or a metal compound. In certain embodiments it is,however, also possible for the reaction to be caused by a thermalactivation of the precursor compound, which e.g. comprises a complex ofa metal atom in oxidation level 0, such as iron (0) pentacarbonyl(Fe(CO)₅). In this case the support material may serve as a nucleationpoint for the reaction of the precursor compounds, which thus does notrequire a reactive compound from the sub- or supercritical solvent. Thesub- or supercritical solvent is however still considered a “reactivesolvent” in terms of the invention. The reactive solvent is preferablyliquid at ambient conditions. It is however also contemplated thatgaseous compounds, e.g. CO₂, may be employed as a supercritical solventin the method of the invention. The reaction may be a reduction or anoxidation, or a thermal activation, and the reactive compound may bemolecules of the reactive solvent or the reactive solvent may comprisefurther, e.g. dissolved, reducing or oxidising compounds. When themolecules of the reactive solvent are themselves reducing the reactivesolvent may be referred to as a “reducing solvent”. Likewise, if thereactive solvent comprises oxidising solvent molecules the reactivesolvent may be referred to as an “oxidising solvent”. The reactivesolvent may be selected from alcohols, ethers, ketones, aldehydes,amines, amides, water and other organic based liquids; preferredreactive solvents are ethanol, methanol, isopropanol, ethylene glycol,water and combinations thereof. Alcohols, e.g. ethanol, methanol andisopropanol, ethylene glycol are generally considered reducing solvents.Oxidising solvents comprise hydrogen peroxide, nitric acid and water, oran oxidising compound, such as KMnO₄, RuO₄, HNO₃, H₂SO₄, OsO₄, may becontained in the reactive solvents. It is noted that certain oxidisingcompounds comprise metal ions that may also be relevant for thecatalytic nanoparticles and may therefore also represent a precursorcompound. Thus, in certain embodiments the precursor compound alsoprovides an oxidising effect. This is particularly relevant when two ormore precursor compounds are required to provide the desired catalyticnanoparticles. The reactive solvent may also comprise a mixture ofsolvents, including reducing solvents with non-reducing solvents oroxidising solvents with non-oxidising solvents. A reducing solvent maybe employed to reduce an oxidised metal component of a precursorcompound to the metal in its metallic form. An oxidising solvent may beemployed to oxidise a partly oxidised metal component of a precursorcompound to a more, e.g. fully, oxidised metal in order to providecatalyst nanoparticles of a metal compound comprising the oxidised metaland one or more partner atoms or ligands. Likewise, reducing solvent maybe employed to reduce an oxidised metal component of a precursorcompound to the metal at a lower oxidation level; such reactions mayalso provide catalyst nanoparticles of a metal compound comprising anoxidised metal and one or more partner atoms or ligands. Yet a furtherembodiment uses two or more different precursor compounds wherein anoxidised metal atom in one precursor compound can reduce an oxidisedmetal atom of another precursor compound. Likewise, ligands of theprecursor compounds may also provide a reducing or oxidising effect.Furthermore, certain solvents may be either reducing or oxidisingdepending on the conditions, e.g. regarding pressure and temperature.

In a certain embodiment, the suspension of the support material and/orthe reactive solvent may also comprise a dispersion agent. In thecontext of the invention a “dispersion agent” is any compound that mayaid in the dispersion of the support material and it may further improvethe processing by minimising undesirable deposition of the support orprepared catalyst in unit operations, such as valves, pumps, mixers,inlets, outlets etc. in the process stream. This is especiallyadvantageous when the process is operating continuously since it allowsthe process to proceed for extended periods of time. A preferreddispersion agent is ethylene glycol, for example present at aconcentration in the range of from about 0.1% to 10%, e.g. such as about1%, about 2%, about 3%, about 4%, about 5%. Ethylene glycol isparticularly advantageous as a dispersion agent when the reactivesolvent is a reducing solvent, such as an alcohol, e.g. ethanol. Otherdispersion agents comprise any non-ionic surfactant, e.g. Triton X-100,or polymeric compounds, such as polyvinyl pyrrolidone, polyoxyethylenesorbitan monolaurate etc.

Solvents generally have a critical point regarding temperature andpressure defining a supercritical regime, which is reached whenexceeding the critical point in the phase diagram. The temperature valueand the pressure value of the critical point are abbreviated “T_(cr)”and “P_(cr)”, respectively, in the context of this invention. In thesupercritical regime distinct liquid and gas phases do not exist, and inthis regime the fluid will have special properties which have manyadvantages for the synthesis of catalyst nanoparticles. Compared toconventional liquid solvents, the high diffusivities and low viscositiesof supercritical or subcritical fluids result in enhanced mass-transfer.The low surface tension of supercritical or subcritical fluids can alsohelp avoiding collapse of the support material. The present inventorshave now surprisingly found that this allows that the high specificsurface area of the support material is efficiently made available fornucleation of catalyst nanoparticles so that aggregation of formednanoparticles can be avoided and the catalyst nanoparticles can bedistributed, as individual catalyst nanoparticles, on the supportmaterial to form a catalytic structure. The properties of supercriticaland subcritical fluids are tuneable by changing the pressure andtemperature. In particular, density and viscosity change drastically atconditions close to the critical point, e.g. at a temperature at orwithin 100° C. below T_(cr) and a pressure at or within 30% belowP_(cr). There are generally no upper limits to the temperature andpressure in the method of the invention. However, it is contemplatedthat the temperature should generally be below 1000° C. and the pressuregenerally be below 1000 bar. In certain embodiments the upper limit ofthe temperature is within 500° C., within 200° C. or within 100° C.above the T_(cr), and the pressure has an upper limit of 2000%, 1000%,500% or 200% of the P_(cr).

In the context of the invention the terms “supercritical” or“supercritical state” refer to the state of a solvent above its criticalpoint regarding temperature (T_(cr)) and pressure (P_(cr)). The reactivesolvent may also be in a subcritical state. The term “subcritical state”generally refers to the state where one or both of the temperature andthe pressure are below the critical point values T_(cr) and P_(cr). Inparticular, in the context of the invention a sub-critical state may beformed when a solvent is exposed to a temperature at or within 100° C.,e.g within 50° C., e.g. within 40° C., 30° C., 20° C. or 10° C., belowthe T_(cr) while the pressure is at or within 30%, e.g. within 25%, 20%,15%, 10%, or 5%, below the P_(cr). When either of the pressure or thetemperature is within these ranges and the temperature or the pressure,respectively, is above the corresponding critical point value thesolvent is also considered to be in a subcritical state. The super- andsubcritical states may also be referred to as super- and subcriticalconditions, respectively. In certain embodiments the state of thereaction solution may be changed between supercritical conditions andsubcritical conditions and vice versa. When the supercritical orsubcritical reactive solvent is admixed with the mixture of the solutionof the precursor compound and the suspension of the support materialbeing under ambient conditions the temperature and pressure of theadmixture will typically drop relative to the temperature and pressureof the reactive solvent due to ambient conditions of the mixture of thesolution of the precursor compound and the suspension of the supportmaterial. However, due to the design of the apparatus the temperatureand pressure of the reaction solution are quickly increased to thedesired values. This allows that the initiation of the reaction of theprecursor compound can be controlled further. As an example ethanol at atemperature in the range of 250° C. to 400° C. at a pressure of 100 barto 300 bar is mixed with the mixture of the solution of the precursorcompound and the suspension of the support material providing atemperature at the mixing point in the range of about 100° C. to about325° C. In other embodiments, the pressure and/or temperature of themixture of the solution of the precursor compound and the suspension ofthe support material is increased, e.g. to subcritical or supercriticalconditions, in particular to the same pressure and temperature as thereactive solvent, prior to admixing with the reactive solvent undersubcritical or supercritical conditions.

The temperature and pressure values of the critical points of solventsare known to the skilled person. Specific examples of critical points ofselected solvents are given in Table 1.

TABLE 1 Critical points of selected solvents Critical temperatureCritical pressure Solvent Formula (° C.) (MPa) Water H₂O 374 22.1Ethanol C₂H₅OH 241 6.14 Methanol CH₃OH 240 8.09 Isopropanol C₃H₇OH 2354.76 Acetone C₃H₆O 235 4.7 Diethylether (C₂H₅)₂O 194 3.68 Carbon dioxideCO₂ 31 7.38

Table 2 provides examples of relevant catalyst nanoparticles andrelevant support materials; Table 2 further provides relevantapplications for catalytic structures with the specified catalystnanoparticles.

TABLE 2 Exemplary catalysts and relevant supports Catalyst nano- Supportparticle example* Reaction Ni MgAl₂O₄ CH₂═CH₂ + H₂ −> CH₃CH₃ (10-100m²/g) Ch₄ + H₂O −> CO + 3 H₂ Pt/Pd/Rh 2 CO + 2 NO −> 2 CO₂ + N₂ V₂O₅SO₂ + ½O₂ −> SO₃ ZnO—Cr₂O₃ Al₂O₃ or Cr₂O₃ CO + 2 H₂ −> CH₃OH Cu/ZnOCO₂ + 3 H₂ −> CH₃OH + H₂O Mn_(x)O_(y) H₂O₂ −> H₂O and ½O₂ Fe/Ru N₂ + 3H₂ −> 2 NH₃ (Haber Process) Ni_(x)O_(y) Natural gas −> Methane TiO₂Photocatalytic activity Ru CO −> CO₂ MoS₂ Hydrogen evolution reactionCo/Fe/Ru/Ni (2n + 1) H₂ + n CO → C_(n)H_((2n+2)) + n H₂O(Fischer-Tropsch reaction) Pt/Pt_(x)RU_(y) Carbon Fuel cells (e.g. DMFC)Pt_(x)Sn_(y)/Ru_(x)Se_(y)O_(z) Co—Mo CO decomposition at 700° C.Cu₈Zn₅Al₂ ZMS-5 Cu—Zn—Al catalyst with high Cu₆₇Zn₅₇Al₂₇ Zeoliteselectivity for hydrogen Cu₆Zn₇Al₂ production through Steam ReformingCu₆Zn₅Al₄ (Avoiding CO poisoning) Cu₅₃Zn₆₃Al₃₃ Cu₄₇Zn₇₇Al₂₇ C₄₇Zn₅₇Al₄₇Cu₄Zn₉Al₂ Cu₄Zn₇Al₄ Cu₄Zn₅Al₆ Cr_(x)O_(y) Al₂O₃ C₄H₁₀ −> C₄H₈ + H₂ (100m²/g) Ni_(x)MO_(y)Sz Al₂O₃ C₄H₄S + 4H₂ −> C₄H₁₀ + H₂S Co_(x)MO_(y)Sz(200-300 m²/g) Pd or Ni Carbon Hydrogenation *Typical specific surfacearea for the indicated support is given in parantheses

Relevant precursor compounds containing platinum are H₂PtCl₆.6H₂O,H₂PtCl₆.xH₂O, PtCl₂, PtCl₄, PtO₂, cis-dichlorobis(pyridine)platinum(II),platinum(II) acetylacetonate (Pt(C₅H₇O₂)₂) (also known as Pt(acac)₂),PtBr₂, PtI₂, dichloro(ethylenediamine)platinum(II) (H₂NCH₂CH₂NH₂)PtCl₂),transplatinum(II)diammine dichloride (Pt(NH₃)₂Cl₂), platinum(IV) oxidehydrate (PtO₂.xH₂O), ammonium hexachloroplatinate(IV) ((NH₄)₂PtCl₆),potassium hexachloroplatinate(IV) (K₂PtCl₆). Relevant precursorcompounds containing ruthenium are RuCl₃, Ru(acac)₃, ruthenium(III)chloride hydrate (RuCl₃.xH₂O), ruthenium iodide (RuI₃), ruthenium(IV)oxide hydrate (RuO₂.xH₂O), ruthenium(III) bromide (RuBr₃), hexaammineruthenium(II) chloride ([Ru(NH₃)₆]Cl₂). Relevant precursor compoundscontaining palladium are Palladium(II) acetate Pd(OAc)₂, Pd(NO₃)₂,PdCl₂, Na₂PdCl₄, (Ethylenediamine)palladium(II) chloride(Pd(H₂NCH₂CH₂NH₂)Cl₂), Palladium(II) iodide (PdI₂), Palladium(II)bromide (PdBr₂), PdO, OPd.xH₂O, Pd(OH)₂, Pd(OH)₄, Palladium(II) nitratedihydrate (Pd(NO₃)₂.2H₂O), Palladium(II) nitrate hydrate(Pd(NO₃)₂.xH₂O), Palladium(II) trifluoroacetate ((CF₃COO)₂Pd),Palladium(II) hexafluoroacetylacetonate (Pd(C₅HF₆O₂)₂, Palladium(II)sulfate (PdSO₄), Palladium(II) cyanide (Pd(CN)₂), Palladium(II)propionate ((C₂H₅CO₂)₂Pd), Palladium(II) potassium thiosulfatemonohydrate (K₂Pd(S₂O₃)₂.H₂O), Dichloro(1,5-cyclooctadiene)palladium(II)(C₈H₁₂Cl₂Pd), Dichlorobis(triethylphosphine)palladium(II)([(C₂H₅)₃P]₂PdCl₂), Ammonium tetrachloropalladate(II) ((NH₄)₂PdCl₄),Potassium tetrachloropalladate(II) (K₂PdCl₄). Relevant precursorcompounds containing gadolinium are gadolinium chloride Gd(III)Cl₃,gadolinium bromide Gd(III)Br₃, gadolinium iodide Gd(III)I₃, gadoliniumflouride Gd(III)F₃, gadolinium(III) chloride hydrate Gd(III)Cl₃.xH₂O,gadolinium(III) nitrate hydrate Gd(NO₃)₃.xH₂O, gadolinium(III)trifluoromethanesulfonate (CF₃SO₃)₃Gd, gadolinium(III) sulfate hydrateGd₂(SO₄)₃.xH₂O, gadolinium(III) sulfate Gd₂(SO₄)₃, gadolinium(III)oxalate hydrate Gd₂(C₂O₄)₃.xH₂O, gadolinium(III) tris(isopropoxide)C₉H₂₁GdO₃, gadolinium(III) carbonate hydrate Gd₂(CO₃)₃.xH₂O,gadolinium(III) hydroxide hydrate Gd₂(OH)₃.xH₂O. Relevant precursorcompounds containing yttrium are yttrium chloride Y(III)Cl₃, yttriumbromide Y(III)Br₃, yttrium iodide Y(III)I₃, yttrium flouride Gd(III)F₃,Y(III) chloride hydrate Y(III)Cl₃.xH₂O, yttrium triflouroacetateY(OOCCF₃)₃, yttrium(III) nitrate hydrate Y(NO₃)₃.xH₂O, Yttriumacetylacetonate Y(C₅H₇O₂)₃ (also known as Y(acac)₃), Yttriumacetylacetonate hydrate Y(C₅H₇O₂)₃.xH₂O, yttrium(III)trifluoromethanesulfonate (CF₃SO₃)₃Y, Yttrium(III) acetate hydrate(CH₃CO₂)₃Y.xH₂O, Yttrium isopropoxide oxide OY₅(OCH(CH₃)₂)₁₃,yttrium(III) carbonate hydrate Y₂(CO₃)₃.xH₂O The platinum, ruthenium,palladium, gadolinium and yttrium precursor compounds are typicallyemployed to provide the respective metallic nanoparticles, although itis contemplated that these may also provide metal compounds comprising apartner atom or ligand.

Table 3 lists relevant iron containing precursor compounds. The ironatoms of the precursors are at oxidation levels 2 or 3. Further ironcontaining precursor compounds are iron (0) pentacarbonyl (Fe(CO)₅),(+)-Iron(II) Lascorbate (C₁₂H₁₄FeO₁₂), Ammonium iron(II) sulfatehexahydrate ((NH₄)₂Fe(SO₄)₂.6H₂O), Ammonium iron(III) citrate(C₆H₈O₇.xFe³⁺.yNH₃), Ammonium iron(III) hexacyanoferrate(II) hydrate(C₆H₆Fe₂N₇O), Ammonium iron(III) oxalate trihydrate((NH₄)₃[Fe(C₂O₄)₃].3H₂O), Ammonium iron(III) sulfate dodecahydrate(NH₄Fe(SO₄)₂.12H₂O), Cyclopentadienyl iron(II) dicarbony) dimer(C₁₄H₁₀Fe₂O₄). It must be understood that the iron atom can readily bereplaced with other metal atoms. In particular, the ligands of the ironcomplexes in Table 3 are relevant as ligands for precursor compounds ofother metal atoms as well.

TABLE 3 Iron containing precursor compounds Iron(II) Iron(III)acetylacetonate (Fe(C₅H₇O₂)₂) acetylacetonate (Fe(C₅H₇O₂)₃) carbonate(FeCO₃) fluoride (FeF₃) chloride (FeCl₂) chloride (FeCl₃) chloridetetrahydrate fluoride trihydrate (FeCl₂•4H₂O) (FeF₃•3H₂O) bromide(FeBr₂) bromide (FeBr₃) iodide (FeI₂) chloride hexahydrate (FeCl₃•6H₂O)D-gluconate dehydrate citrate (C₆H₅FeO₇) ([HOCH₂[CH(OH)]₄CO₂]₂Fe•2H₂O)ethylenediammonium sulfate citrate tribasic monohydrate tetrahydrate(C₆H₅FeO₇•H₂O) (FeSO₄•NH₃(CH₂)₂NH₃SO₄•4H₂O) hydroxide (Fe(OH)₂)hydroxide (Fe(OH)₃) fumarate (C₄H₂FeO₄) iodate (Fe(IO₃)₃) gluconatehydrate nitrate (Fe(NO₃)₃•9H₂O) ((C₆H₁₁O₇)₂Fe•xH₂O) fluorosilicatenitrate nonahydrate (FeSiF₆•6H₂O) (Fe(NO₃)₃•9H₂O) oxalate dihydrateoxalate hexahydrate (FeC₂O₄•2H₂O) (Fe₂(C₂O₄)₃•6H₂O) molybdate (FeMoO₄)oxo acetate perchlorate hydrate (C₁₂H₂₄Fe₃O₁₆•ClO₄•xH₂O) perchlorate(Fe(ClO₄)₂•6H₂O) perchlorate (Fe(ClO₄)₃) perchlorate hydrate perchloratehydrate (Fe(ClO₄)₂•xH₂O) (Fe(ClO₄)₃•xH₂O) nitrate (Fe(NO₃)₂•6H₂O)phosphate (FePO₄) lactate hydrate phosphate dihydrate([CH₃CH(OH)COO]₂Fe•xH₂O) (FePO₄•2H₂O) sulfate (FeSO₄•7H₂O) phosphatetetrahydrate (FePO₄•4H₂O) sulfate heptahydrate p-toluenesulfonatehexahydrate (FeSO₄•7H₂O) ((CH₃C₆H₄SO₃)₃Fe•6H₂O) sulfate hydratepyrophosphate (Fe₄(P₂O₇)₃) (FeSO₄•xH₂O) sulfide (FeS)Ethylenediaminetetraacetate sodium salt hydrate([(O₂CCH₂)₂NCH₂CH₂N(CH₂CO₂)₂]FeNa•xH₂O) tetrafluoroborate hexahydratesulfate (Fe₂(SO₄)₃•9H₂O) (Fe(BF₄)₂•6H₂O) trifluoromethanesulfonatesulfate hydrate (C₂F₆FeO₆S₂) (Fe₂(SO₄)₃•xH₂O) tartrate (Fe₂(C₄H₄O₆)₃)trifluoroacetylacetonate (C₁₅H₁₂F₉FeO₆) arsenate (FeAsO₄)

Exemplary set-ups of reactors for use in the present invention areillustrated in FIG. 1 and FIG. 2. Thus, FIG. 1a shows a set-up where themixture of the solution of the precursor compound and the suspension ofthe support material is provided to an injector 1 via a feed pump 2. Thereactive solvent is provided via a solvent pump 3 to a heater 5. Boththe mixture with the support material and the precursor compound may becooled in a cooler 4 before being supplied to a mixer 6. The cooler 4may serve to prevent that the pump or other heat sensitive parts areheated. In the mixer 6 the step of admixing the mixture of the solutionof the precursor compound and the suspension of the support materialwith the supercritical or subcritical reactive solvent takes place. Theadmixture is provided to the first section of the reactor tube 7 whichfirst section comprises a heater. The reactor tube may comprise acooling section 8 for liquefying the reaction solution. The set-up has apressure release valve 9 allowing collection of the catalytic structurein a collection vessel 10. The set-up in FIG. 1b adds a further reactantpump so that the system has a first reactant pump 21 and a secondreactant pump 22. This set-up does not have an injector and thereactants, i.e. the solution of the precursor compound and thesuspension of the support material are mixed before they are admixedwith the reactive solvent in the mixer 6. It is also contemplated to mixthe solution of the precursor compound, the suspension of the supportmaterial and the reactive solvent simultaneously in an appropriate mixer(not shown). In a further reactor set-up as shown in FIG. 2a the reactortube comprises a further inlet, so that a further precursor compound,support material, an oxidising or reducing component, or a component toactivate the support material may be supplied via reactant pump 23. Inthis set-up a support may be supplied via a reactant pump 21 and acomponent to activate the support material via reactant pump 23; theprecursor compound may be supplied to the, now activated, supportmaterial via reactant pump 23. Alternatively reactant pump 23 may supplya further precursor compound to provide a catalytic structure withlayered or mixed catalytic nanoparticles of two different catalystmaterial, or it may provide a catalytic structure with two differenttypes of catalytic nanoparticles. This set-up is specifically intendedfor a reactor capable of synthesising core-shell structures. In yet afurther design (FIG. 2b ) an additional solvent pump is added to theset-up illustrated in FIG. 2a . The additional solvent pump 24introduces a solvent at a location downstream of the inlet to thereactor tubes of reactant pump 23, e.g. into a mixer 61 capable ofmixing the stream from the reactor tube with the stream from reactantpump 23 and the additional solvent pump 24. This set-up is alsospecifically intended for a reactor capable of synthesising core-shellstructures. For example, after the first synthesis of particles on asupport (in reactor tube 7), the suspension is cooled down in cooler 8,mixed with a new reactant from reactant pump 23, and hit by a new hotstring of solvent from the additional solvent pump 24 and subsequentlymatured further in reactor tube 71.

The invention will now be explained in the following non-limitingexamples. As will be evident to the skilled person variations arepossible without deviating from the invention.

EXAMPLES Example 1

A catalytic structure of the invention was prepared as follows.

A mass of 724 mg of a platinum precursor (H₂PtCl₆.6H₂O) was preciselyweighed on a micro scale in order to get precise concentration andweight ratio (Pt/C) in the final synthesis solution. This resulted in apure platinum weight of 273 mg. The mass of the carbon support wasdetermined by using the desired weight-ratio of Pt/C that for 10 wt %carbon support would be 27.3 mg, which was weighed subsequently. The tworeactants respectively were mixed in separate beakers with 100 mL ofethanol as a reducing solvent, which dissolves the platinum precursorsalt, but leaves the carbon support agglomerated. The carbon support wasprior to the experiment sonicated for 5-10 min using an ultrasonic hornto disperse the C-support and ensure access to larger surface areas.

The two reactants were mixed in an injector with a volume of 200 mL,which was mounted in connection with the feeding pump of the supercritical flow apparatus (FIG. 1a ). An injector was used because thefeeding pumps of the apparatus were sensitive to handling the smallcarbon support particles, which cause fast degradation of the packingseals. The feeding pump feeds pure solvent to the injector, hencesupplying a cold reactant line of the dissolved platinum salt and carbonsupport. The cold reaction line was mixed abruptly with preheatedsolvent at a pressure of ˜200 bar (adjustable) resulting in a mixingtemperature of ˜300° C. (adjustable) in the super critical regime of thereducing ethanol solvent. High heating rates can be obtained by mixingthe cold reaction line and super critical solvent, which are importantfor obtaining fast nucleation and reaction uniformity. The criticaltemperature and pressure are solvent dependent, and hence tuneable byusing different solvents. The obtained product is tuneable by usingtemperature and pressure, thus control of morphology, crystallinity,size and uniformity of the particles was obtained. This resulted inhomogenous nanoparticles with a narrow size distribution, which isadvantageous for the catalytic properties of the nanoparticles. Thesynthesis itself can also be performed below the super critical point ofthe solvent, i.e. in a subcritical state of the solvent, which againwill affect the size and crystallinity of the product particles. TheC-support that is present in the super- or sub-critical media preventsthe nanoparticles from agglomerating, as these attach directly onto thecarbon support.

The apparatus has a vertical reactor tube through which the producttravels down while maturing the particles further to enhancecrystallinity, and thereby generating more well-defined particles. Thereaction solution was subsequently cooled indirectly by flowing water onthe outside of the reaction tubing to a temperature where the solutionliquefies. The pressure of the system was relieved by a valve (Pressurerelease valve), and the solvent, including the nanoparticles synthesiseddirectly onto the carbon support, i.e. the catalytic structure, werecontinuously tapped.

The catalytic structure of the nanoparticles supported by carbon wascentrifuged, and the product settled at the bottom of beakers.Subsequently the product was washed by a solvent, in this case ethanol,two times after which the final product could be dried on a glassbeaker.

A modified version of the apparatus employed will assure that thereaction will not start prematurely in the injector, as the tworeactants can be mixed just prior to meeting the superheated, e.g.supercritical, solvent. Furthermore, the injector can be omitted due tothe use of robust industrial feeding pumps. Another improved apparatusdesign comprises the addition of several reaction inlets at differentpositions, which allow for greater flexibility.

Example 2

The syntheses of the catalytic platinum and platinum-rutheniumnanoparticles directly onto various carbon supports are here reported.The syntheses were performed in the supercritical regime, which for thesolvent ethanol is above 241° C. and 61.4 Bar.

The reactions were carried out in a purpose built synthesis flow systemwhich can withstand the harsh conditions of the supercritical fluids.The schematic shown in FIG. 1a is a simplified version of theexperimental set-up in which the general parts are illustrated.

The precursor compound, e.g. platinum precursor (H₂PtCl₆.6H₂O; 721 mg),was prior to the experiment dissolved in 50 mL of ethanol leading to asolution of the metal precursor. The support material, e.g. graphenesupport (67.1 mg), was dispersed in 50 mL of ethanol, and sonicated fora few minutes to minimise agglomeration and obtain an optimaldispersion.

The metal precursor solution and the dispersed carbon support were mixedin an injector. The system was pressurised to a synthesis pressure of180-190 Bar. At the mixing point the cold reactant stream was mixed withthe super critical preheated solvent, ethanol, leading to a mixingtemperature of 258-267° C. The rapid increase in the temperature leadsto fast homogenous nucleation resulting in monodisperse nanoparticles.The continuous flow of the produced nanoparticles on the carbon supportwas withdrawn from the system using a pressure release valve.

The synthesis products were characterized using powder X-ray diffraction(PXRD), scanning electron microscopy (SEM) and transmission electronmicroscopy (TEM). The catalytic activity of the powders wascharacterized using cyclic voltammetry (CV) in a three-electrodeelectrochemical set-up. FIG. 3 shows electron micrographs of exemplarycatalytic supports, FIG. 4 shows an PXRD diffractogram of a catalyticstructure, and FIG. 5 shows CV-plots of the catalytic structures of theinvention compared to a commercial catalytic structure.

The PXRD of the crystalline catalyst materials results indiffractograms. One example is shown in FIG. 4 illustrating the resultfor nanoparticles prepared from H₂PtCl₆.6H₂O with 20 wt % graphene; afit is shown along with the PXRD data for the size determination. Fromthe Bragg-angles, the material and crystal structure can be found whilethe line broadening provides information about the particle (grain)sizes. FIG. 3 shows TEM and SEM images of samples of the preparedcatalytic structures. Thus, FIG. 3a shows a TEM image of Pt-particles onCarbon Black XC-72; FIG. 3b shows a TEM image of Pt-particles onKetjenblack EC600JD; FIG. 3c shows a TEM image of Pt-particles ongraphene (10 wt % graphene); the sizes of the nanoparticles are seen torange from 5 nm to 20 nm. FIG. 3d shows a TEM image of Pt-particles ongraphene, with a lower Pt-loading (20 wt % graphene) than FIG. 3c ; thesizes of the nanoparticles range from 2 nm to 8 nm. FIG. 3e shows a SEMimage of Pt-particles on graphene (10 wt % graphene); both small andlarger nanoparticles are seen. SEM of the powders provide an overview ofthe particle composition on the support, as well as the materialdistribution using energy dispersive x-rays (EDX). TEM provides highermagnified images of the particles, and thus the particle sizedistribution can be found (FIG. 3).

The catalytic activity is found from CV measurements (see FIG. 5), wherea mixture of powder and Nafion (proton conducting ionomer) is dispersedon a glassy-carbon electrode. This electrode is used as a workingelectrode in a three-electrode system, whereas a pure Pt electrode isused as a counter electrode and a Mercury-Mercurous Sulphate electrodeis used as a reference. The electrochemical surface area is found byCO-stripping in a 0.5 M H₂SO₄ solution, while a 1 M MeOH+0.5 M H₂SO₄solution is used for the electrochemical activity of the powders.

Table 4 presents the most important synthesis of Pt or PtRunanoparticles onto support materials. Mainly three carbon supports havebeen used; Graphene nanopowder (1-3 layers of graphene, diameter approx.10 μm) and two types of carbon black pallets (Carbon Black XC-72 andKetjenblack EC600JD), but also MWCNTs and reduced graphene oxide havebeen used. The stated weight percentage of the support is given withrespect to the amount of pure Pt, or Pt and Ru in the precursor. TEMimages and CV measurements have only been taken/made for a few of theseexperiments, while all have been analysed with PXRD. The premixing ofsupport and precursor in the injector causes a redox reaction and asmall part of the precursor reacts and solidifies on the support beforeinjection in the reactor, resulting in large particles larger than 20nm. After injection, the remaining (major part) of the precursor reactsand solidifies on the support as small nanoparticles in the order of 5nm, in the controlled environment. This division in sizes of thecatalyst nanoparticles is seen from the PXRD data, and results in twosize-readouts. FIG. 3e shows a SEM image where both small and largeparticles are present. Separate inlets of precursor and support preventthis preinjection reaction on the support.

TABLE 4 Most important syntheses of Pt-nanoparticles onto supports in areactor set-up with one inlet to the reactor tube. T (° C.)/P Particlesize Precursor Support Solvent (bar) (nm) H₂PtCl₆•6H₂O 10 wt % Ethanol248-252° C./ 20 nm/ graphene 185-190 bar 7 nm H₂PtCl₆•6H₂O 20 wt %Ethanol 258-267° C./ 21 nm/ graphene 180-190 bar 6 nm H₂PtCl₆•6H₂O 30 wt% Ethanol 278-287° C./ 23 nm/ graphene 170-190 bar 6 nm H₂PtCl₆•6H₂O 20wt % Ethanol 271-285° C./ 22 nm/ rGO 190-220 bar 6 nm H₂PtCl₆•6H₂O 20 wt% Ethanol 250-280 bar 35 nm/ XC-72 4 nm H₂PtCl₆•6H₂O 20 wt % Ethanol275-276° C./ 22 nm/ EC600JD 190-230 bar 4 nm H₂PtCl₆•6H₂O 30 wt %Ethanol 280-283° C./ 21 nm/ EC600JD 190 bar 3 nm H₂PtCl₆•6H₂O 40 wt %Ethanol 283-295° C./ 20 nm/ EC600JD 170-205 bar 3 nm Pt(acac)₂ 5 wt %Ethanol + ~300° C./ graphene acetylacetone 200-300 bar Pt(acac)₂ 10 wt %Ethanol + ~300° C./ graphene acetylacetone 200-300 bar Pt(acac)₂ 10 wt %Ethanol + ~300° C./ XC-72 acetylacetone 200-300 bar Pt(acac)₂ 20 wt %Ethanol + ~300° C./ XC-72 acetylacetone 200-300 bar Pt(acac)₂ 10 wt %Ethanol + ~300° C./ MWCNTs acetylacetone 200-300 bar Pt(acac)₂ 20 wt %Ethanol + ~300° C./ MWCNTs acetylacetone 200-300 bar Pt(acac)₂ + 10 wt %Ethanol + ~300° C./ Ru(acac)₃ XC-72 acetylacetone 200-300 barPt(acac)₂ + 20 wt % Ethanol + ~300° C./ Ru(acac)₃ XC-72 acetylacetone200-300 bar

The grain size investigations show that using Pt(acac)₂ as precursorresulted in almost only large Pt particles (>15 nm), as opposed to usingH₂PtCl₆.6H₂O as precursor where the size distribution was largelydominated by 4-6 nm Pt particles. Thus, the precursor compound may beselected to control the size of the catalyst nanoparticles.

In certain experiments the reactive ethanol solvents were supplementedwith small concentrations of H₂O, H₂O₂ or H₂SO₄ (<5%). The components ofthe reactive solvent can enhance the nanocarbon activation.

The catalytic activity of the prepared catalytic structures was foundfrom CV measurements analysing approximately 4 mg powder of each sample.A 1 M MeOH+0.5 M H₂SO₄ solution was used to measure the electrochemicalactivity of the powders, and the resulting CV curves for some of thepowders are shown in FIG. 5, compared to commercial catalyst particlesfrom Johnsson Matthey (HiSpec 13100), which were tested under identicalconditions. FIG. 5 thus shows the results for three different catalyticstructures produced within the supercritical flow reactor (details ofthe synthesis are given in FIG. 5). The graph shows the current dividedby the electrochemical surface area vs. the applied the potential. Thegraph of FIG. 5 clearly shows the potential of the produced catalyticstructure, showing higher currents at any relevant potential than thecommercial reference. The catalyst synthesised onto graphene shows themost promising characteristics.

Example 3

The synthesis of the catalytic platinum nanoparticles directly ontocarbon supports is here reported. The syntheses were performed in thesupercritical regime, which for the solvent ethanol is above 241° C. and61.4 Bar.

The reactions were carried out in a purpose built synthesis flow systemwhich can withstand the harsh conditions of the supercritical fluids.The schematic shown in FIG. 1b is a simplified version of theexperimental set-up in which the general parts are illustrated.

The platinum precursor (H₂PtCl₆.6H₂O; 724 mg) was prior to theexperiment dissolved in 50 mL of ethanol leading to a solution of themetal precursor compound. The graphene support was dispersed in 50 mL ofethanol, and sonicated for a few minutes to minimise agglomeration andobtain an optimal dispersion.

The metal precursor compound solution was pumped through reaction pump1, whereas the dispersed carbon support was pumped through reaction pump2 into the pressurized system at 180-220 Bar. At the mixing point thecold reactant streams mix with the super critical preheated reactivesolvent, ethanol, leading to a mixing temperature above thesupercritical temperature of ethanol. The exact temperature was notrecorded. The rapid increase in the temperature leads to fast homogenousnucleation resulting in monodisperse nanoparticles. The continuous flowof the produced nanoparticles on the carbon support was withdrawn fromthe system using a pressure release valve, which also kept the systempressurized.

The synthesised products, the catalytic structures, were characterizedusing PXRD, SEM and TEM as described previously.

The separate inlets prevented a premature redox reaction and thus thatthe precursors solidify on the carbon substrates before entering thereactor and the supercritical regime. Thus no large nanoparticles werefound from the PXRD measurements, backing up our thesis from the resultsof Example 2. The grain sizes are shown in Table 5.

TABLE 5 Pressure conditions and particle size of preferred embodimentsfor synthesizing Pt nanoparticles on supports. P Particle size PrecursorSupport (bar) (nm) H₂PtCl₆•6H₂O 10 wt % graphene 180-220 Impurity phaseH₂PtCl₆•6H₂O 20 wt % graphene 130-230 4 H₂PtCl₆•6H₂O 20 wt % XC-72180-220 5 H₂PtCl₆•6H₂O 30 wt % XC-72 180-220 5 H₂PtCl₆•6H₂O 20 wt %EC600JD 180-220 4 H₂PtCl₆•6H₂O 30 wt % EC600JD 180-220 4

Example 4

The synthesis of the catalytic platinum nanoparticles directly ontoKetjenBlack EC600jd (“KB”) as a carbon support material is herereported. The syntheses were performed in the supercritical regime,which for the solvent ethanol is above 241° C. and 61.4 Bar.

The reactions were carried out in a purpose built synthesis flow systemwhich can withstand the harsh conditions of the supercritical fluids. Aschematic drawing is shown in FIG. 1 b.

General Description of the Experiments

The platinum precursor (H₂PtCl₆.6H₂O; 717 mg, 0.00138M) was prior to theexperiment dissolved in 100 mL of ethanol leading to a solution of themetal precursor. The carbon support (270 mg for a 50:50 Pt:C ratio) wasdispersed in 100 mL Ethanol and with 1 vol % Ethylene Glycol (EG), andsonicated for 10 minutes to achieve good particle dispersion. The EGimproves the carbon dispersion in the solvent, and 1 vol % has proved tobe enough, minimising the chance of a pump-stops.

The metal precursor solution was pumped through reaction pump 21,whereas the dispersed carbon support was pumped through reaction pump 22into the pressurized system at a constant pressure of 100-300 Bar (+/−10bar). Both pumps 21,22 were kept at a flow of 10 mL/min, though otherexperiments have been performed in the range of 5-15 mL/min. The solventheater was kept at 450° C. while the vertical heater was kept constantat 250° C.-425° C. The precursor and support streams meet and propermixing of the two is ensured through static mixers installed within thepipes. At the mixing point 6 the cold reactants streams mix with thesupercritical preheated solvent, ethanol, leading to a mixingtemperature near or above the supercritical temperature of ethanol,easily adjusted by the solvent pump 3 flow rate. The exact mixingtemperature was recorded and kept at 257° C. (+/−5° C.), which is wellabove the supercritical temperature. The rapid increase in thetemperature leads to fast homogenous nucleation resulting inmonodisperse nanoparticles, which are further matured down the verticalheater before being cooled down. The continuous flow of the producednanoparticles on the carbon support was withdrawn from the system usinga pressure release valve 9, which also keeps the system pressurized. Thefirst and last 25% of the synthesised product is discarded to minimiseconcentration variations that may occur at start and end of theexperiment to dilution of the reactant strings. It is noted that whenthe process is set up to operate on a continuous basis the amount ofcatalyst prepared as the first and last part that may be discarded willbe insignificant compared to the remaining product.

The separate inlets prevent the reaction to occur prematurely and thusthe precursor from solidifying on the carbon substrates before enteringthe reactor and the supercritical regime. In order for proper mixing ofthe precursor and support, a static mixer is used before reaching thehot solvent. In order for proper mixing with the hot solvent, variousmixing geometries can be used, such as cross-, vortex- or opposingflow-mixing, illustrated in FIG. 6.

While the Ethylene Glycol in the support mixture improves the carbonsupport dispersion, further dispersion improvement can be achieved byactivating the carbon support. In our case, both boiling in 8M HNO₃ for8 hours as well as stirring in 2M H₂O₂ for 48 hours were tried, bothimproving the carbon dispersion due to an activation of the carbonsurfaces. This surface functionalisation, however, also affects theinteraction with the negatively charged Pt-salt in the supercriticalsolvent, and thus neither the HNO₃- nor the H₂O₂-activation improved thecatalytic properties of the produced catalyst. Also dispersion agents,such as Polyvinylpyrrolidone (PVP), have been tried.

The synthesis products were characterised using powder X-ray diffraction(PXRD), scanning electron microscopy (SEM), scanning transmissionelectron microscopy (STEM), thermogravimetric analysis (TGA) (Pt:Cratio) and half-cell cyclic voltammetry (CV) (mass activity (MA) andelectrochemical surface area (ECSA).

The electrochemical surface area (ECSA) was measured from CO-adsorptionin 0.1M HClO₄. The mass activity (MA) was found from the oxygenreduction reaction (ORR) at 0.9 V, sweeping with 50 mV/s in oxygensaturated 0.1M HClO₄ solution, while rotating the glassy carbonelectrode at 1600 rpm.

Size Control

The graphs in FIG. 7 show the vertical heater (maturing) temperature(T_(v)) and pressure (p) dependence on the Pt particle size (PXRDmeasured), and illustrate a very precise size control in the 1-5 nmrange. Specifically, in FIG. 7 The flow of both pump 21 and 22 were keptat 10 mL/min, while the solvent pump 3 was adjusted to give a mixingtemperature of 257° C. The temperature dependence (a) was measured atp=300 bar, while the pressure dependence (b) was measured at T_(v)=400°C. The electrochemical measurements (ECSA and MA) of these particles areshown in FIG. 8, indicating an increase in both surface area andactivity with decreasing particle size. For comparison Johnson MattheyHispec13100 with an average size of 4.2 nm is included in the graph inFIG. 8. Long-term (stress) tests have not been performed, though it isexpected that smaller Pt particles will deteriorate faster than bigger,due to agglomeration, sintering and dissolution of the Pt.

The distribution of the Pt particles on the KetjenBlack is shown in FIG.9, for two sizes of particles. For both, a good Pt particle distributionis seen with no unattached, agglomerated Pt.

Pt:C-Ratios

The Pt:C ratio of the synthesised catalyst is easily regulated from theconcentrations of the Pt precursor and KetjenBlack (or the pump flowrates). The graphs in FIG. 10 show the correlation of the Pt:KB ratio,ECSA and MA of Pt particles synthesised on KB. Specifically, thecatalyst structures were synthesised with a constant H₂PtCl₆concentration (0.00138 M) and varying KB content (filled markers), orthe catalyst structures were synthesised with a constant KB amount (270mg in 99 ml EtOH/1 ml EG) and varying H₂PtCl₆ concentration (hollowmarkers). A small decrease in both ECSA and MA is seen with increasingPt concentration, which can be ascribed to better dispersion of the Ptparticles on the support for lower Pt concentrations.

Other Carbon Supports

Other carbon nanoparticles than KetjenBlack have been tested as catalystsupport materials, including different types of carbon nanotubes andgraphene flakes, to increase both activity as well as durability of thecatalyst. Electron micrographs of some of these synthesised catalystmaterials are shown in FIG. 11. Specifically, FIG. 11 in (a) and (b)show Pt particles on graphene with ratio 50:50 (Pt:G); (c) shows Ptparticles on MWCNTs (8-13 nm diameter) with ratio 50:50 (Pt:CNT), and(d) shows Pt particles on MWCNTs (8-13 nm diameter) with ratio 20:80(Pt:CNT). Several types of graphene have been used, though the mostsuitable has been thermally exfoliated and acid refined flakes with manydefects and voids (grade AO-1 from www.graphene-supermarketcom), with asurface area of 700 m²/g (shown in FIGS. 11(a) and (b). Both ECSA and MAwere comparable to Pt on KB (ECSA=50-90 m²/gPt, MA=0.2-0.3 A/mgPt),though especially the MA measurements were complicated by the fact thatgraphene likes to stack when dried on the electrode, causing oxygenstarvation during the measurement for a lot of the Pt particles. Thushigher MAs are to be expected.

Also for the CNTs several types have been used, where mainly diameter,length and surface treatment were the varying parameters, allmulti-walled (MWCNT). A good Pt distribution on HD plasma treated tubeswas achieved, though the low surface area of the tubes (diameter 13-18nm, surface area below 100 m²/g) resulted in an excess amount of Ptunable to synthesise on the carbon surface, instead causing agglomeratedPt particles. Still ECSAs of 50-80 m²/gPt and MAs over 0.2 A/mgPt wereachieved. Thinner MWCNTs (diameter 8-15 nm, surface area approximately233 m²/g) were also tried, with lengths of 0.5-2 μm, and ECSA and MA inthe same range. As Pt agglomerations were seen (carbon surface area muchlower than that of KetjenBlack EC600jd; 1400 m²/g) varying Pt:C ratioswere tried, as shown in FIGS. 11(c) and (d), with lower amounts of Pt,thus reducing the amount of agglomerated Pt particles.

While no durability tests of the catalyst have been performed yet,thermogravimetric analysis (TGA), from which the exact Pt:C ratio isfound (the catalyst is weighted while all carbon is burned), shows theeffect on durability for a graphitised support. FIG. 12 shows the TGAanalysis of three different catalysts with different carbon supportnanomaterials. The solid curve shows time is progressing as a functionof temperature. A 5% oxygen atmosphere was let into the chamber after 20minutes at 150° C. (after degassing in N₂ at 150° C. and below). Thecatalyst with KetjenBlack as support is seen to burn at the lowesttemperature, whereas the MWCNT burned at the highest. The MWCNT catalysthad 40 wt % Pt while the Graphene and KetjenBlack catalyst had 50 wt %Pt.

Pt synthesis with new short single-walled tubes and short multi-walledwith a diameter below 8 nm (both tubes with a surface area of 400-500m²/g) are planned as future experiments.

All results presented for the tubes, were for H₂O₂-activated tubes, asnon-activated tubes tends to agglomerate and settle quickly. The H₂O₂activation is fairly mild, to preserve the tube-structure.

Example 5

The synthesis of the catalytic platinum nanoparticles directly ontoKetjenBlack EC600jd (KB) is here reported. The syntheses were performedin the supercritical regime with ethanol as the reactive solvent. Aschematic drawing of the set-up is shown in FIG. 1 b.

The platinum precursor (H₂PtCl₆.6H₂O; 357 mg, 0.00138M) was prior to theexperiment dissolved in 50 mL of ethanol leading to a solution of themetal precursor. The carbon support (135 mg for a 50:50 Pt:C ratio) wasdispersed in 50 ml absolute EtOH or in a mixture of Ethanol and EthyleneGlycol (EG) (1-25 vol % EG), and sonicated for 10 minutes to achievegood particle dispersion. The EG improves the carbon dispersion in thesolvent, while also acting as a reducing agent.

The metal precursor solution was pumped through reaction pump 21,whereas the dispersed carbon support was pumped through reaction pump 22into the pressurised system at 290-310 Bar. The solvent heater was keptat 450° C. while the vertical heater was kept at 400° C. At the mixingpoint the cold reactants streams mix with the super critical preheatedsolvent, ethanol, leading to a mixing temperature near or above thesupercritical temperature of ethanol, easily adjusted by the pump flowrates. The exact temperature was recorded. The rapid increase in thetemperature leads to fast homogenous nucleation resulting inmonodisperse nanoparticles. The continuous flow of the producednanoparticles on the carbon support was withdrawn from the system usinga pressure release valve, which also kept the system pressurised.

The synthesis products were characterized using PXRD, SEM, STEM, TEM andhalf-cell cyclic voltammetry (CV) as described previously.

Table 6 shows the results when varying the Ethylene Glycol content inthe carbon support solution. A good dispersion was observed at only 1vol % EG, minimizing the chance of a pump-stop compared to 0% EG. ThePXRD and ECSA results also show the most promising results with 1 vol %EG.

Table 7 shows the results when varying the mixing temperature,controlled by the solvent flow. A preferred mixing temperature of about260° C. is observed, and a Scanning Transmission Electron Micrograph ofthis product revealed good Pt distribution on the carbon support andvery little Pt particle agglomeration.

Lower mixing temperatures are seen to produce particles with smallermean size, however when close to the critical temperature (T_(c)=243° C.for EtOH) larger fluctuations in pressure and hence temperature andparticle size are observed, resulting in a lower ECSA.

TABLE 6 Size and ECSA of preferred embodiments for synthesising Ptnanoparticles on ketjenblack EC600jd, with various amounts of EthyleneGlycol. All with H₂PtCl₆ × 6H₂O as precursor and Ethanol as solvent, andsynthesized at T_(sol) = 450° C., P = 300 bar and T_(mix) = 270° C.Ethylene Glycol 1% 2% 5% 10% 25% Mean size  5 nm  5 nm  5 nm  5 nm  6 nm(PXRD) ECSA 65 m²/g 50 m²/g 54 m²/g 46 m²/g 40 m²/g (Hupd)

TABLE 7 Size and ECSA of preferred embodiments for synthesizing Ptnanoparticles on ketjenblack EC600jd, with 1 vol % Ethylene Glycol atvarious T_(mix). All with H₂PtCl₆ × 6H₂O as precursor and Ethanol assolvent, and synthesized at T_(sol) = 450° C., P = 300 bar and T_(mix)controlled by the solvent flow rate. T_(mix) 230° C. 249° C. 257° C.270° C. 286° C. Mean size  4 nm  4 nm  5 nm  4 nm  4 nm (PXRD) ECSA 52m²/g 46 m²/g 69 m²/g 57 m²/g 40 m²/g (Hupd)

The invention claimed is:
 1. A method of preparing a catalytic structurehaving catalyst nanoparticles, the method comprising the steps of:providing a solution of a precursor compound in a liquid solvent atambient conditions; providing a suspension of a support material havinga specific surface area of at least 1 m²/g in a liquid solvent atambient conditions; optionally sonicating the suspension of the supportmaterial; mixing the solution of the precursor compound and thesuspension of the support material; providing a reducing reactivesolvent or an oxidizing reactive solvent in a supercritical orsubcritical state; admixing the mixture of the solution of the precursorcompound and the suspension of the support material in the supercriticalor subcritical reactive solvent to form a reaction solution; injectingthe reaction solution into a reactor tube via a first inlet; allowing areaction of the precursor compound in the supercritical or subcriticalreactive solvent in the reactor tube to form the catalyst nanoparticleson the support material to provide the catalytic structure; andwithdrawing the catalytic structure from the reactor tube via an outlet,which is downstream from the first inlet.
 2. The method according toclaim 1, wherein the reaction takes place under continuous conditions.3. The method according to claim 1, wherein the reactor tube comprisesone or more additional inlets downstream of the first inlet.
 4. Themethod according to claim 1, wherein the reactive solvent has atemperature at or within 100° C. below, or above the temperature of thecritical point (T_(cr)) of the reactive solvent and the reactive solventis at a pressure at or within 30% below, or above the pressure of thecritical point (P_(cr)) of the reactive solvent.
 5. The method accordingto claim 1, wherein the catalyst nanoparticles are metallic and themetal is selected from the group consisting of a transition metal, alanthanide, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Gd, Y, Zr, Nb, Mo,Ru, Rh, Pd, Ag, Cd, Pt, Au, Ir, W, Sr or a mixture thereof.
 6. Themethod according to claim 1, wherein the catalyst nanoparticles comprisea metal compound.
 7. The method according to claim 6, wherein the metalcompound comprises a metal atom and a partner atom selected from thegroup consisting of groups 13, 14, 15 or 16 of the periodic table of theelements, and/or a ligand molecule.
 8. The method according to claim 1,wherein the suspension of the support material and/or the reactivesolvent comprises a dispersion agent.
 9. The method according to claim1, wherein the reactive solvent is ethanol, methanol, isopropanol,ethylene glycol or a combination thereof.
 10. The method according toclaim 1, wherein the ratio of the precursor compound to the supportmaterial is in the range of 1:100 to 100:1.
 11. The method according toclaim 1, wherein the reactive solvent comprises a component to activatethe support material.
 12. The method according to claim 1, wherein thesupport material is a carbon material selected from the group consistingof graphene, reduced graphene oxide, graphene oxide, carbon nanotubes(CNT), carbon black or carbon aerogel.
 13. The method according to claim1, wherein the support material is selected from the group consisting ofaerogels, ceramic materials, metals, metal alloys, zeolites, tungstencarbide, metal oxides and metal sulphides.
 14. The method according toclaim 1, wherein the size of the catalyst nanoparticles is in the rangeof about 1 nm to about 50 nm.
 15. The method according to claim 1,wherein the catalyst nanoparticles are monodisperse and have diameterswith a standard deviation up to 50% of the diameter.
 16. The methodaccording to claim 1, wherein the distance between the first inlet andthe outlet coupled with the flow rate of the reaction solution in thereactor tube provides a residence time for the reaction solution flowingthrough the reactor tube, which residence time is in the range of 2seconds to 10 minutes.